On 2020-05-02 11:59 a.m., Eric V. Smith wrote:
On 5/2/2020 7:18 AM, Alex Hall wrote:
On Sat, May 2, 2020 at 12:39 PM Henk-Jaap Wagenaar
<wagenaarhenkj...@gmail.com <mailto:wagenaarhenkj...@gmail.com>> wrote:
Of course, there are other ways of writing this code, but imagine
this for a database interface where a save normally returns the
saved object (inspired by Django)
```
try:
x, y = Foo.save()
except ValueUnpackingError:
// oh... this means saving failed (it returned None instead)
// let's return a sensible response
return ExceptionToResponse(FooStateToException(foo.state))
```
My concern is that this is risky code. If `Foo.save()` has a bug
somewhere inside (or maybe it calls back to your own code which has a
bug) it could raise `ValueUnpackingError` for different reasons, and
then that gets caught and misleads people into thinking that Foo.save
returned None which leads to all sorts of frustration and confusion.
This code:
```
result = Foo.save()
if result is None:
return ExceptionToResponse(...)
x, y = result
```
is not sexy, but it's explicit, safe, and it targets the real problem
precisely.
Creating these new exceptions would encourage people to write code
that will bite them later.
I agree that we don't want to encourage code like the above example
with catching the hypothetical ValueUnpackingError. I contend that
even if we added ValueUnpackingError, no one would ever write code
that used it, but should instead do what they do now: unpack it after
checking it (as Alex's example shows). Of course, that's a horrible
API, but sometimes you have to call horrible APIs. The fact that the
API is bad doesn't mean we should add a way to handle it poorly.
The only reason I'd want it to exist (and I don't) is to see it in the
REPL. But for that, there's the exception message and the traceback,
so a unique exception for this would be useless to me.
how about:
result = Foo.save()
try:
x, y = result
except ValueUnpackingError:
return ...
this would also handle generators (and, the unpacking operation should
wrap the generator's ValueUnpackingError (if any) in a RuntimeError so
it doesn't conflict with the unpacking operation's API)
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